A new technique called centrifugal elutriation has been applied to the separation of platelets from whole human blood. This technique is based on the use of the Beckman Elutriator rotor. The principle involves counterflow centrifugation in which the tendency of particles to sediment in a centrifugal field is balanced by a liquid medium flowing in the opposite direction. We have modified the original Beckman system to adapt it more specifically to our purposes. Platelets have been isolated in high yield from two to five ml. of blood by this method. They are completely free of red and white blood cells and appear morphologically intact. Their functional integrity remains to be tested. From these preliminary results, it appears that centrifugal elutriation can be used as a new laboratory method for the separation of purified platelets. Based on this, we have presented a detailed description of an elutriation system scaled up for clinical use. This system promises to be capable of isolating platelets from units of whole blood in quantities sufficient for clinical transfusions. The expected advantages of this proposed scaled-up system over present methods of collecting platelets are the speed of collection, the increased yield of platelets, the purity and concentration of the isolated platelets and the expected reduced cost of operation of the system.